The present invention relates to the field of marine seismic exploration. More particularly, the invention relates to a web structure towed behind a seismic vessel or deployed in water to support sensors and other marine seismic equipment, and for providing redundant signal transmission paths between the equipment and the vessel.
Marine seismic exploration vessels tow acoustic energy sources such as compressed air guns through the water. The vessel also tows one or more seismic streamer cables along a selected survey line. Each streamer contains multiple hydrophones which receive the reflected energy emitted by the energy source from subsurface geologic formations and boundaries. Each hydrophone generates signals representative of the reflected energy, and such signals are stored and processed to provide data representative of subsurface geologic structures. For bottom cable systems, geophones are laid on the sea floor along seismic cable lines. Data from adjacent seismic lines are processed to construct an overall geologic image. To account for vessel movement, data recording and processing calculations require time and position correlations for each active component of the seismic data gathering system.
Representative marine seismic systems were described in U.S. Pat. No. 3,133,262 to Strange et al. (1964) wherein a pair of wires transmitted signals from each hydrophone group to a connecting network, and in U.S. Pat. No. 3,441,902 to Savit (1969) wherein the array length and configuration of arrays in a streamer cable were changed to create different receiver patterns. In U.S. Pat. No. 4,319,347 to Savit (1982), a large number of sensor units and seismic arrays were established along the length of each seismic cable and the signals from adjacent vertical lines were combined to produce a cross section or composite geophysical survey of the target area. Additionally, U.S. Pat. No 4,545,039 to Savit (1985) disclosed the concept of operating two seismic vessels in parallel along a survey line to obtain three survey lines on each pass.
Advanced three dimensional seismic processing techniques require large quantities of data to provide high data resolution. The cell size of a processed data sampling determines the spatial frequency desired, and smaller cell sizes provide higher spatial frequency. As described in U.S. Pat. No. 5,511,039 to Flentge (1996), higher spatial frequencies provide higher seismic data resolution which correspondingly increases the resulting stratigraphic information quality. The ability to gather and process additional data facilitates reconstruction of the geologic image at any point in time.
Three dimensional marine seismic surveys typically incorporate multiple seismic lines comprising a plurality of uniformly spaced streamer receivers in parallel lines as described in U.S. Pat. No. 5,598,378 to Flentge (1997). The reflected signals are recorded and displayed in parallel traces along each streamer, and moveout and angularity corrections adjust the collected data. The streamers typically range between three and eight kilometers long, and tail buoys attached to the free streamer ends incorporate radar reflectors and navigation and acoustic transponders. Hydrophones in a streamer are typically wired together to form receiver groups regularly spaced along the streamer. The interval between receiver groups typically ranges between 12.5 and 6.25 meters, and the spacing between adjacent streamers typically comprises 50 meters. The relatively large spacing between adjacent streamers reduces streamer fouling due to crosscurrents, wind, and vessel course changes.
U.S. Pat. No. 4,726,315 to Bell et al. disclosed booms for towing multiple streamers behind a seismic vessel. When multiple streamers are towed, diverters typically pull the side positioned streamers outwardly from the vessel, and anchor buoys can maintain tension against each streamer. If the streamers are not adequately separated, the streamers can become entangled when the streamers are deployed, used, or retrieved. Ocean currents, waves and vessel course changes can vary the streamer orientation during use. Streamer entanglement risks increase in long streamers and in arrays having multiple streamers.
Variations in the lateral spacing between adjacent streamers due to environmental forces and vessel course changes introduces variables in the collected data. The spacing between receiver groups along a streamer and between streamers is critical to the analysis of geophysical data, and is important to surface and subsurface criteria. The surface criteria regarding receiver groups is important to the accurate recordation of reflected seismic signals and to various noise interfering with such signals. The acoustic wavefield is more accurately represented when the receiver groups are more closely spaced together. The amount of "aliasing" experienced, defined as the natural approximations made in sampling processes for time or space, also depends on the spacing of receiver groups. In coarse sampling with a greater distance between receiver groups, more of the wavefield is aliased and the data quality is reduced.
The amount of aliasing is critical to the successful attenuation of noises interfering with the desirable reflected seismic energy. Aliasing effects can modify the recorded signals and are especially disruptive for reflections from shallow or steeply dipping geologic formations such as the flanks of geologic structures or salt bodies. Although conventional streamers can provide fine sampling along an individual streamer line, the relatively large spacing between adjacent streamers limits effective noise attenuation processes and reduces the fidelity of reflected signals at higher frequencies and steep dips.
The quality of seismic maps created from processing techniques such as stacking, normal-moveout correction, dip moveout correction and migration also depends partially on the bin size. Smaller bin sizes are prevented by the safe spacing limits between adjacent seismic streamers. Subsurface sampling criteria regarding receiver groups depends on several other factors, and receiver group spacing determines the bin size into which recorded data are collected and processed to form subsurface images. For complex and steeply dipping subsurface formations, receiver group spacings are preferably smaller to preserve the desired signals. Smaller spacings are required to adequately perform image formation processes such as Dip Moveout and Time or Depth Migration processes.
In addition to the between-streamer spacing limitations necessary to avoid streamer fouling, the reliability of conventional streamer systems is limited to the data path transmission capabilities of each streamer. Data is frequently transmitted to recording instrumentation on the seismic vessel through an electrical wire or optical fiber attached to or integrally formed within each streamer. If the signal communication medium is damaged along the streamer path, all of the data from the streamer will be lost and seismic data collection operations must be discontinued to permit repair of the damaged streamer.
For these reasons, a need exists for an improved data collection system in marine seismic exploration operations towed behind a vessel or deployed along the sea floor. The system should enhance the accuracy of data processing and the data collection and processing reliability of the geophysical operations.